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Defined contribution plan matching percent of compensation.

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- Definition

Employee Benefits Disclosure [Line Items]

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- Definition

Estimated Number Of Participants

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- Definition

Retirement Plans, Minimum Employment Period For Benefits

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- Definition

Total pretax intrinsic value of options exercised and the fair value of restricted stock vested based on the actual stock price at time of exercise/vesting.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Shares Vested

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- Definition

share based compensation arrangement by share based payment award, number of shares transferred Number of shares transferred from one Equity Plan to Another

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- Definition

Company shares held in retirement plan.

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- Definition

The maximum number of shares authorized for issuance under the deferred compensation arrangement as of the balance sheet date.

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-Name Accounting Standards Codification
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- Definition

Unrecognized cost of unvested share-based compensation awards.

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- Definition

Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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- Definition

The carrying amount as of the balance sheet date of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Person shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Benefits Disclosure [Line Items]
Minimum employment period for benefits	30 days
Employee Stock Ownership Plan Shares held \| shares	24,100,000
Postemployment benefits liability \| $	$ 20.2	$ 19.5
Compensation, Percentage	100.00%
Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Unrecognized compensation cost related to unvested equity awards \| $	$ 100.6
Aggregate fair value of the restricted equity awards that vested during the period \| $	162.7	130.5	$ 77.0
Employee Stock Option
Employee Benefits Disclosure [Line Items]
Unrecognized compensation cost related to unvested equity awards \| $	0.2	0.7	1.6
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Unrecognized compensation cost related to unvested equity awards \| $	$ 0.9
Vesting period of grants to date	11 months
Total pretax intrinsic value of options exercised and restricted stock vested \| $	$ 3.2	$ 2.2	$ 3.0
Executive Deferred Compensation Plan
Employee Benefits Disclosure [Line Items]
Common shares reserved for issuance under executive deferred compensation plan \| shares	11,100,000
2003 Directors Equity Incentive Plan \| Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized \| shares	0
2017 Directors Equity Incentive Plan \| Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized \| shares	500,000
Equity Incentive Plan Twenty Ten [Member]
Employee Benefits Disclosure [Line Items]
Shares authorized \| shares	0
share based compensation arrangement by share based payment award, number of shares transferred \| shares	4,000,000
Equity Incentive Plan Twenty Fifteen [Domain]
Employee Benefits Disclosure [Line Items]
Shares authorized \| shares	17,000,000
Performance Based Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Estimated Number Of Participants \| Person	50
Performance Based Restricted Equity Awards \| Insurance Operating Results \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards \| Insurance Operating Results \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	250.00%	250.00%	250.00%	250.00%	250.00%	250.00%
Performance Based Restricted Equity Awards \| Investment Results \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards \| Investment Results \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	200.00%	200.00%	200.00%	200.00%	200.00%	200.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies [Member] \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount				100.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies [Member] \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount				200.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount				0.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment [Member] \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount		85.00%	85.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment [Member] \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount		150.00%	150.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount		0.00%	0.00%
Defined Contribution Pension Plan 401k
Employee Benefits Disclosure [Line Items]
Matching contributions \| $	$ 106.8	$ 97.3	$ 86.8
Defined Contribution Pension Plan 401k \| Upper Limit
Employee Benefits Disclosure [Line Items]
Matching contribution to defined contribution pension plan	6.00%
Cash [Member]
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	40.00%
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	60.00%
Vesting tranche one \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	3 years
Vesting tranche two \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	4 years
Vesting tranche three \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	5 years